An operand is one of two parts of an operation.
An operation, for example, is 4+5. Here, the operands are 4 and 5, while the operator is +. The operator is what happens to ("operates on") the operands.
A similar word relation in the English language (though not the same) would be employee and employer. The employer is the one who employs the employee. The operator is what operates on the operand or operands. I say "operand or operands" here because there are unary (single-operand operations), binary (two-operand operations), ternary (three-operand operations) and combinations of operators which yield something like x=4+5. You have the 4+5 operation, which results in 9, then you have the x=9 operation, which results in assigning the value of 9 to the variable x.